UNDERSTANDING FIBER OPTICS

UNDERSTANDING FIBER OPTICS UNDERSTANDING FIBER OPTICS

12345Attenuation and wavelengthLight is gradually attenuated when it is propagated along the fber. The attenuation value is expressed in dB/km (decibel per kilometer). It is a function of the wavelength (), i.e. of the color (frequency) of the light. That means that the operating wavelength to transmit a signal in an optical fber is not any wavelength.It corresponds to a minimum of attenuation.The following graph gives the linear attenuation as a function of the wavelength:linear

attenuation

(dB/km)The operating wavelengths, which light sources have been developed for, are 850 nm (nanometers) and 1300 nm in multimode, and 1310 nm and 1550 nm in singlemode.Example: For a 850 nm operating wavelength, there is a 3 dB light attenuation after 1 km propagation (according to the graph). 3 dB mean that 50% of the light has been lost. wavelength (nm) Multimode fberMultimode fber850 nm 1300 nm 1550 nm core: This central section, made of silica, is the light transmitting region of the fber. cladding: It is the frst layer around the core. It is also made of silica, but not with the same composition as the core. This creates an optical waveguide which confnes the light in the core by total refection at the core-cladding interface. coating: It is the frst non-optical layer around the cladding. The coating typically consists of one or more layers of a polymer that protect the silica structure against physical or environmental damage.p2WHAT IS AN OPTICAL FIBER ?Fiber structure and fber types - An optical fber is made of 3 concentric layers as described on the following sketch:corecladdingcoatingThis type of fber is called multimode because light rays travel through the fber following different paths called modes.In that case, the fber is called singlemode because only one mode is propagated. It travels straight through the fber. The core diameter is typically 9 m.CHARACTERISTICS OF OPTICAL FIBERcore diameter in microns (m)cladding diameter in microns (m)Singlemode fber9 / 12550 / 125 or 62,5 / 125cladding diameter in microns (m)core diameter in microns (m) Singlemode fberUNDERSTANDING FIBER OPTICS UNDERSTANDING FIBER OPTICSThe main advantages of fber optics are the followings: Lower loss: Optical fber has lower attenuation than copper conductors, allowing longer cable runs and fewer repeaters. Increased bandwidth: The high signal bandwidth of optical fber provides a signifcantly greater information-carrying capacity. Typical bandwidths for multimode fbers are between 200 and 600 MHz.km, and > 10 GHz.km for singlemode fbers. Typical values for electrical conductors are 10 to 25 MHz.km. Immunity to interference: Optical fbers are immune to electromagnetic and radio frequency interference and also emit no radiation themselves. No detection: Standard fber optic cables are dielectric, so they cannot be detected by any type of detector. Electrical isolation: Fiber optics allows to transmit information between two points at two different electrical potentials, and also next to high voltage equipments. Decreased size and weight: Compared to copper conductors of equivalent signal-carrying capacity, fber optic cables are easier to install, require less duct space, and weight about 10 to 15 times less. multimode caseWHY TO CHOOSE FIBER OPTICS ?Information (A, B or C) is propagated in fber according to N modes (paths), which deform it, as if it were duplicated N times (for example on the diagram above, the mode 3 path is longer than the mode 2 path, which is itself longer than the mode 1 path). If information is too close one to the other, there is a risk of mixing, and it will not be recoverable at the exit of fber. That is why it is necessary to space it suffciently, i.e. to limit the fow.Information (A, B or C) is propagated in fber according to only one mode, therefore is not deformed. It is thus possible to closer the information, i.e. to obtain a much more important fow. mode 1 mode 2 mode 3A B CA B CA B CA B CCCC AAA BBBAAABBBCCCA B CA B CBandwidth is a measure of the data-carrying capacity of an optical fber. It is expressed as the product of frequency and distance. For example, a fber with a bandwidth of 500 MHz.km (Mega-hertz kilometer) can transmit data at a rate of 500 MHz along one kilometer.Bandwidth in singlemode fbers is much higher than in multimode fbers:Bandwithp3 singlemode caseUNDERSTANDING FIBER OPTICS UNDERSTANDING FIBER OPTICSp41 - Fusion spliceThis operation consists in directly linking two fbers by welding with an electric arc, by aligning best possible both fber cores. The specifc device to make this fusion is called a fusion splicer.Advantages:HOW TO LINK TWO OPTICAL FIBERS ? This linking method is fast and relatively simple to make. The light loss generated by the welding, due to an imperfect alignment of the cores, remains very weak.Drawbacks: This type of link is relatively fragile (in spite of a protection of fusion by a heat-shrinkable tube). It is a permanent link. It is necessary to invest in a fusion splicer.2 - Use of connectorsIn this case, it is necessary to terminate a connector at each end of the fbers to be connected. The two fbers can then be connected by connecting the two connectors together.Advantages: This type of connection is robust. The type of connector can be chosen according to the application feld of the system. Connection is removable. It is possible to connect and disconnect two fbers hundreds to thousands times without damaging the connectors.Drawbacks: The implementation is longer than fusion, and requires an experiment as well as specifc tools. The light loss due to connection is higher than in the splicing solution.There are two ways of linking two optical fbers:250 m coated fber250 m coated fberThe coated fber, i.e. the two active layers (core and cladding) and the protective coating, has an external diameter of 250 microns. It is very fragile. It is thus necessary to build cables to reinforce this fber and to make it easier to handle. There is a great number of different cable constructions (see below some examples).FIBER OPTIC CABLESsingleway cables:outer sheath0,3 mm outer diametermultiway cables:5,5 mm outer sheath0,9 mm buffer kevlar (strength members)0,9 mm buffer kevlar (strength members)0,9 mm outer diameterUNDERSTANDING FIBER OPTICS UNDERSTANDING FIBER OPTICSCONNECTORS TYPESFiber termination in the ferruleWhatever the needed connector, the frst step consists in inserting the fber in a ferrule, to allow to simplify the fber handling with less risk to damage it. This ferrule is generally made of ceramics and is manufactured with high precision machining process.fber optic cable glue barefber(125 m )ceramic ferruleThe different steps to terminate the fber in the ferrule are the followings: fber stripping to keep only the active layers (core and cladding). fber epoxy bonding in the ceramic ferrule. The fber is introduced into the ceramic ferrule hole whose diameter is very precise, and adjusted to that of fber. fber cleaving at the ceramic ferrule surface. polishing of the end of the ceramic ferrule. Using lapping flms of increasingly fne grains, the fber surface is perfectly well polished, and all the awkward residual particles have been eliminated.Butt joint technologyPrinciple: The principle of the butt joint connectors consists in putting in physical contact the two ceramic ferrules. To realign perfectly fbers face to face, we use an alignment sleeve generally in ceramics.The light passes thus directly from one fber to the other. ceramic ferrulefbersleeveDefects:The alignment of fbers is never perfect, some light is lost when going from one side to the other. This loss can be high according to the residual defects of alignment or polishing:transverse shift air gap (bad contact)angular shift bad polishingp5UNDERSTANDING FIBER OPTICS UNDERSTANDING FIBER OPTICSp6Lens technology (expanded beam technology)Principle: The principle of the expanded beam connectors consists in placing a lens at the exit of each fber, in order to widen the beam by collimating it - i.e. by creating light beams parallel to the optical axis. In this confguration, there is no more physical contact between the two optical fbers.lensesconnector shellexpanded beamDefects:In this case it is the alignment of the two shells one to the other which will guarantee that the collimated beam going out from the frst lens will be well refocused through the second lens. The precision of the mechanical interface parts of the connector is highly important. As for the butt joint connectors, transverse and angular shifts, and also bad polishing will generate losses.Lens connectors characteristics: The light loss generated by connection (called Insertion Loss) is more important than in the previous case, due to the presence of the lenses and sometimes also of windows (approximately 1 to 1,5 typical dB). This type of connection is much less sensitive to pollution because the beam is much larger than the one that goes out directly from a fber. A dust at the interface of two butt joint connectors will create a much higher loss than located at the interface of two lens connectors. Lens connectors examples:CTOS (2/4 channels) CTOL (8 channels)Butt joint connectors characteristics: The loss of light generated by connection (called Insertion Loss) is low (approximately 0,3 dB typical). This type of connection is sensitive to pollution (dust, mud...). If a dirtiness stays between the two ceramic ferrules, a big part of the light can be lost.ferruleButt joint connectors examples:ST (singleway) RNJOP (4 channels)UNDERSTANDING FIBER OPTICS UNDERSTANDING FIBER OPTICSCHARACTERISTICS OF FIBER OPTIC LINKSA fber optic interconnection system can generally be described by the following sketch:It is made of the following elements : emission systemIt contains a light source. To obtain a well functioning system, it is important to know : the type of light source (LED or laser). the operating wavelength (1300 nm for example). the light source power. the interface connector type at the light source output. one or more optical patchcords The pachcords are defned by: the fber optic cable described by its length, the number of channels, the fber type (singlemode, multimode 50/125 or 62,5/125). the connectors terminated at both ends of the cable. the whole patchcord is characterized by a loss named Insertion Loss, expressed in dB. This loss gives the quantity of lost light when introducing this patchcord in the transmission line. It includes the linear attenuation of the cable and the loss due to the connectors. for singlemode patchcords, another important parameter, called Return Loss, represents the quantity of light which is refected by the line and which returns in the direction of the source. The laser sources used in singlemode applications can be very sensitive to this phenomenon.Connection Optical patchcordEmission Receptionoptical linkP inP backP outThe Insertion Loss of the link is: PI = 10 log (Pout / Pin)The Return Loss of the link is: RL = 10 log (Pback / Pin) reception systemIt contains a receiver which has a photosensitive surface, and which converts the light into electric signal. It is important to know: its sensitivity, i.e. the minimal quantity of detectable light. the interface connector type at the receiver input.For more information about our fber optic product range, see the document Solutions for Fiber Optic Connection .p7Amphenol is the fber optic expert you need to solve your problem !UNDERSTANDING FIBER OPTICSNORTH AMERICAAMPHENOL PCD, Inc.2 technology DrivePeabody, MA 01960Telephone: 1-978-532-8800Fax: 1-978-532-6800AMPHENOL AEROSPACE OPERATIONSAMPHENOL INDUSTRIAL OPERATIONS40-60 Delaware StreetSidney, New York 13838-1395 - USATelephone: 1-607-563-5011Fax: 1-607-563-5157AMPHENOL BACKPLANE SYSTEMS18 Celina AvenueNashua, New Hampshire 03063 - USATelephone: 1-603-883-5100 - Fax: 1-603-883-0171AMPHENOL CANADA CORPORATION20 Melford Drive - Scarborough, Ontario M1B 2X6CanadaTelephone: 1-416-291-4401Fax: 1-416-292-06471870 boul. des Sources, Suite 204Pointe Claire, Quebec H9R 5N4 - CanadaTelephone: 1-514-630-7242 ext. 225Fax: 1-514-630-7697AMPHENOL FIBER OPTIC PRODUCTS-RICHARDSON OPERATION1778 North Plano Road, Suite 212Richardson, Texas 75081Telephone: 1-972-744-9801Fax: 1-972-744-9022AMPHENOL FSI1300 Central Expwy N, Suite 100Allen, TX 75013 U.S.A.Toll-Free: 800-472-4225 - Fax: 214-547-9344info@fbersystems.comEUROPEAMPHENOL SOCAPEXMIL-AERO & INDUSTRIAL Business Unit948 Promenade de lArve - B. 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